Fossil fuels such as coal, oil and natural gas recently used in thermal power plants etc. release a vast amount of carbon dioxide into the atmosphere by burning. Increase of the released carbon dioxide in the atmosphere will deteriorate global environment by causing global warming etc., in addition, it will significantly affect the human society through the occurrence of natural disaster and heavy damage of crops, because increase of carbon dioxide in the atmosphere is the cause of a frequent drought, heavy rain and floods.
Thus, the development of the technology has been long-awaited which can fix carbon dioxide with less energy to decrease the amount of carbon dioxide released into the atmosphere. And as a simple, safe and effective method, a method has been investigated and is about to be utilized to fix carbon dioxide in the atmosphere using photosynthesis of plant microorganisms which is caused by irradiation of sunlight etc. However, in the construction of the conventional photosynthetic culture systems using plant microorganisms, since their culture baths need to ensure a certain volume of fluid, after photosynthesis proceeds to some extent, light required for photosynthesis does not reach the whole solution sufficiently. Therefore, methods have been used to make photosynthetic culture baths shallow or stir the fluid in the bath so that light will reach the whole solution.
For example, the amount of carbon dioxide in burning gas released from thermal power plants is as vast as about 5000 t/day at 500000 kW output power of burning natural gas, and when burning coal or oil, carbon dioxide emission will be further increased. In such a situation, in order to fix carbon dioxide released from thermal power plants, the systems which are as compact as possible and need less energy for the fixation are desirable. In order to solve these problems, improvements are required in the carbon dioxide fixing technology which are now in use globally such as enhancement of the carbon dioxide fixing ability of photosynthetic culture baths and systems including an optical system, enhancement of efficiency, increase of controllability of the products of photosynthetic reaction.
However, in view of the fact that an effective depth of the culture bath for photosynthesis is only several cm, one of the above methods, in which photosynthetic culture baths are made shallow, has a problem that the area of the photosynthetic culture systems must be extremely large-scale in order to ensure sufficient volume of culture solution. And the other method, in which the fluid is stirred, has also a problem that the whole culture bath is not made good use of since the fluid always subjected to photosynthesis is restricted within the limits where light can reach.
Further, in the conventional photosynthetic culture baths utilizing natural sunlight, since the surface of the solution which is a light-receiving surface is irradiated with an intensive sunlight, light intensity more than needed is wasted and it causes some troubles. In addition, with the progress of photosynthesis, the number of the cells is increased, which prevents light from reaching the depths of the fluid.